"We still can't say how many government employees and private passengers were killed, but there were heavy human losses."

At the blast site, the back of the colourfully decorated bus was torn apart, leaving a mass of snarled metal.

The injured, who included two policemen, were moved to Peshawar's Lady Reading Hospital. Local television footage showed locals carrying victims to the hospital in private vehicles.

US warning

The attack came one day after a remote-controlled bomb killed at least 15 people outside a madrassa, a religious school, in the southwestern city of Quetta.

Pakistan, a country of 180 million people, sits on the frontline of the US-led war on al-Qaeda and since July 2007 has been gripped by local Taliban-led attacks, concentrated largely in the northwest.

In the last five years, attacks blamed on armed groups have killed more than 5,000 people according to a tally by the AFP news agency.

Pakistan's relations with Washington are in disarray, and for the last six months, since US air strikes killed 24 Pakistani soldiers along the border, Islamabad has imposed a blockade on NATO supplies crossing overland into Afghanistan.

On Thursday, Leon Panetta, the US defence secretary, warned Pakistan that the US was running out of patience over Islamabad's refusal to do more to eliminate safe havens for fighters who attack US troops fighting a 10-year war against the Taliban in Afghanistan.